A circuit breaker is an automatically-operated electrical switch which protects an electrical circuit from damage caused by overload or short circuit. In contrast to a fuse, a circuit breaker can be reset to resume normal operation. Many different technologies are used in circuit breakers. One technology includes the field of low-voltage current-limiting Molded Case Circuit Breakers (MCCBs). One of the primary functions is limitation of fault current during a short circuit. When a short circuit occurs, the MCCB is expected to stop the flow of current as quickly as possible to protect conductors and electrical devices in the circuit downstream from the MCCB. The measures of an MCCB's current-limiting ability include the time duration of the fault current, the peak instantaneous let-through current (Ip), and the Joule integral, i.e. J i2dt, where is the instantaneous let-through current and t is time, integrated over the time duration. When an MCCB is declared by the manufacturer to “current-limiting” there are defined maximum limits for Ip and the Joule integral. However, it is generally advantageous to minimize all 3 of these measures in current-limiting MCCBs, to provide the best possible protection of the downstream circuit.
MCCBs typically contain one or more pairs of electrical contacts that close to allow current to flow and open (“break”) to stop the flow of current. The interruption of a flow of a short circuiting current results in an electromagnetic repulsion between a stationary and a movable contact arm causing the arms to separate. During a short circuit event there are very high currents, which means that much inductive and capacitive circuit energy must be dissipated when an MCCB opens and interrupts the fault. When the contacts open, this energy causes an electric arc to form between the contacts. The energy dissipation causes hot conductive plasma near the contacts that allows current to continue to flow.
In order to stop the current flow, MCCBs typically contain metal splitter plates to absorb energy, cool the arc, and reduce the conduc-tivity of the gases. This causes in increase in voltage across the arc, which in turn acts counter to the System voltage, so the flow of current is reduced and eventually stopped. But arc resistance is a function not only of arc conductivity, but also of the length of the arc.
It is extremely important to increase the length of the arc as quickly as possible during a short circuit, to increase the arc voltage and stop the flow of current quickly. Because of this, MCCBs usually have blow-apart contacts, in which the extremely high currents from the short circuit cause magnetic fields, repelling the contacts from each other. Because the blow-apart motion is independent of the operating mechanism motion, blow-apart contacts are able to open much faster during a short circuit than the operating mechanism is able respond. During normal switching operations, the contacts are opened and closed by a toggle spring operating mechanism, by moving a handle.
Alternatively the mechanism can trip and open the contacts automatically. Typically an MCCB includes a trip unit that senses overload currents and responds by actuating the tripping Operation of the mechanism. The trip unit may include a bimetallic strip which is bent and releases a spring-loaded trip-lever if a threshold current is exceeded. Since the heating is fairly slow, another mechanism may be employed to handle large surges from a short circuit. A small electromagnet consisting of one or more conductor loops around a piece of iron will pull an iron armature instantly in case of a large current surge. Alternatively many MCCBs have electronic trip units that contain current sensors, microprocessors, and electromechanical devices that actuate the tripping operation of the mechanism.
There are several methods that have been used for increasing the separation speed of blow-apart contacts. First, there is the simple reverse loop, as shown in FIGS. 1 and 2. An example is the Siemens MCCB catalog number 3VL1716-1 DA33. In a reverse loop, a fixed conductor with a fixed contact is parallel to a moving contact arm. This creates parallel conductors with current flow in opposite directions, resulting in magnetic repulsion of the conductors.
Second, there is the double-blow-apart contact arms concept, shown in FIGS. 3 and 4. This is similar to the reverse loop, but here instead of a fixed contact, both contacts are attached to movable contact arms that mutually repel each other. This essentially doubles the speed and acceleration of contact separation. An example is the Siemens MCCB catalog number MLFB 3VL3725-3DC36.
Third, there is the reverse loop combined with a return loop on the other side of the moving contact arm, as shown in FIGS. 5, 6, and 7. An example is the Moeller NZM Nl. The return loop is parallel to the contact arm. Current flow is in the same direction in both the contact arm and the return loop, therefore the two conductors mutually attract each other. This provides an incremental improvement in speed and acceleration compared to the simple reverse loop.
Fourth, there is the rotating, double-breaking contact system, shown in FIGS. 8, 9 and 10. This concept is described, for example, in Siemens European Patent EP 0174904B1, also described in U.S. Pat. No. 4,649,247. An example is the Schneider MCCB type NS250. There is a rotating contact arm and 2 pairs of contacts that break essentially simultaneously, essentially doubling the speed and acceleration of contact separation, compared to a single pair of contacts. The double-breaking System in this example is used in combination with reversed loops.
Fifth, ferromagnetic material, steel for example, is sometimes used in combination with blow-apart contact systems, to intensify the magnetic field and increase the force on the contact arm. A wide variety of arrangements are possible with ferromagnetic material. Four simplified examples are shown; steel under the fixed contact (FIGS. 11 and 12), a partial slot motor (FIGS. 13 and 14), a full slot motor (FIGS. 15 and 16), and long legs on the arc splitter plates (FIGS. 17 and 18). In all of these methods of rapidly separating the contacts, whether by using conducting loops or ferromagnetic material, a magnetic field causes forces in the contact arm. It should be observed that in each of these cases the magnetic field also has a second benefit; it creates a force in the electric arc that pushes the arc into the splitter plates.
DE 27 20 736 discloses a current limiting device having a movable contact vigorously moved in the open circuit position by an electromagnetic repulsion device at the appearance of a short-circuit current. A retarding member is mechanically linked to the movable contact to delay the re-closing of the contact and to prevent a re-closing before tripping of the circuit breaker.
DE 23 38 637 discloses a contact arrangement for a circuit breaker with double blow-apart contact arms. Each movable contact arm is provided with a contact to a fixed arm. A third contact is provided between the two movable arms.
Another circuit breaker is disclosed in DE 15 63 842.